1. Field of Art
The disclosure generally relates to the field of electronic paper displays. More particularly, the invention relates to pen tracking and low latency display updates on electronic paper displays.
2. Description of the Related Art
Several technologies have been introduced recently that provide some of the properties of paper in a display that can be updated electronically. Some of the desirable properties of paper that this type of display tries to achieve include: low power consumption, flexibility, wide viewing angle, low cost, light weight, high resolution, high contrast, and readability indoors and outdoors. Because these displays attempt to mimic the characteristics of paper, these displays are referred to as electronic paper displays (EPDs) in this application. Other names for this type of display include: paper-like displays, zero power displays, e-paper, bi-stable and electrophoretic displays.
A comparison of EPDs to Cathode Ray Tube (CRT) displays or Liquid Crystal Displays (LCDs) reveal that in general, EPDs require less power and have higher spatial resolution; but have the disadvantages of slower update rates, less accurate gray level control, and lower color resolution. Many electronic paper displays are currently only grayscale devices. Color devices are becoming available although often through the addition of a color filter, which tends to reduce the spatial resolution and the contrast.
Electronic Paper Displays are typically reflective rather than transmissive. Thus they are able to use ambient light rather than requiring a lighting source in the device. This allows EPDs to maintain an image without using power. They are sometimes referred to as “bi-stable” because black or white pixels can be displayed continuously and power is only needed to change from one state to another. However, some devices are stable at multiple states and thus support multiple gray levels without power consumption.
While electronic paper displays have many benefits, a problem is that most EPD technologies require a relatively long time to update the image as compared with conventional CRT or LCD displays. A typical LCD takes approximately 5 milliseconds to change to the correct value, supporting frame rates of up to 200 frames per second (the achievable frame rate is typically limited by the ability of the display driver electronics to modify all the pixels in the display). In contrast, many electronic paper displays, e.g. the E Ink displays, take on the order of 300-1000 milliseconds to change a pixel value from white to black. While this update time is generally sufficient for the page turning needed by electronic books, it is problematic for interactive applications like pen tracking, user interfaces, and the display of video.
One type of EPD called a microencapsulated electrophoretic (MEP) display moves hundreds of particles through a viscous fluid to update a single pixel. The viscous fluid limits the movement of the particles when no electric field is applied and gives the EPD its property of being able to retain an image without power. This fluid also restricts the particle movement when an electric field is applied and causes the display to be very slow to update compared to other types of displays.
When displaying a video or animation, each pixel should ideally be at the desired reflectance for the duration of the video frame, i.e. until the next requested reflectance is received. However, every display exhibits some latency between the request for a particular reflectance and the time when that reflectance is achieved. If a video is running at 10 frames per second and the time required to change a pixel is 10 milliseconds, the pixel will display the correct reflectance for 90 milliseconds and the effect will be as desired. If it takes 100 milliseconds to change the pixel, it will be time to change the pixel to another reflectance just as the pixel achieves the correct reflectance of the prior frame. Finally, if it takes 200 milliseconds for the pixel to change, the pixel will never have the correct reflectance except in the circumstance where the pixel was very near the correct reflectance already, i.e. slowly changing imagery.
In some electronic paper displays, annotation is possible by adding an input sensor layer on top of or underneath the display. These types of electronic paper displays work like a writing tablet. A pen or a stylus is used to activate the pixels on writing surface of the electronic paper display, thus acting like a pen or pencil writing or making annotations on a piece of paper. However, because of the limited speed at which the image can be updated, the EPDs are not effective at showing pen tracking in real time. The key requirements of pen tracking are update speed and contrast, which generally conflict with each other on electronic paper displays. For instance, drawing a light gray line takes shorter time than drawing a black line on some EPDs.
It would therefore be highly desirable to enable both high speed and high contrast on current electronic paper displays, thus allowing for real-time pen tracking.